Alkaline earth metal carbonate powders such as a barium carbonate powder, a strontium carbonate powder and a calcium carbonate powder can be employed for preparing a dielectric ceramic powder. For instance, the barium carbonate powder is employable for the preparation of barium titanate, the strontium carbonate powder is employable for the preparation of powdery strontium titanate, and the barium carbonate powder and calcium carbonate powder are employable for the preparation of powdery barium calcium titanate. The dielectric ceramic powder is utilized for producing a dielectric ceramic layer of a multilayer ceramic capacitor.
Since it is required to provide an electronic device with a smaller size, a multilayer ceramic capacitor is required to have a smaller size. In order to manufacture a multilayer ceramic capacitor having a smaller size, a dielectric ceramic layer should have a less thickness. In order to produce a dielectric ceramic layer having a less thickness, it is required to provide a fine dielectric ceramic powder having a uniform composition.
For the purpose of preparing a fine dielectric ceramic powder (such as powdery strontium titanate, powdery barium titanate, or powdery barium calcium titanate), it is necessary to prepare a fine barium carbonate powder, a fine strontium carbonate powder, a fine calcium carbonate powder, and a fine titanium dioxide powder. In consideration of the necessity of the fine powders, processes for preparing a fine barium carbonate powder, a fine strontium carbonate powder, a fine calcium carbonate powder, and a fine titanium dioxide powder have been studied, and already disclosed in the following patent publications.
Patent Publication 1 (Japanese Patent Provisional Publication (Tokuhyohei) 11-514961) discloses a process for preparing a fine alkaline earth metal carbonate powder which comprises the steps of introducing gaseous carbon dioxide into an aqueous alkaline earth hydroxide solution preferably in the presence of a crystalline growth-inhibitor selected from a group consisting of an ammonium salt of a specific carboxylic acid and an alkylammonium salt of a specific carboxylic acid to produce alkaline earth metal carbonate particles, applying shearing force and friction to the produced alkaline earth carbonate particles at a relatively high rate in a homogenizer under high working pressure, recovering thus treated particles, and drying the recovered particles. Patent Publication 1 describes that the process gives a fine strontium carbonate powder having a BET specific surface area of 3 to 50 m2/g and comprising at least 90% of a powder having a diameter of 0.1 to 1.0 nm, preferably a diameter of 0.2 to 1.0 μm, and a fine barium carbonate powder having a BET specific surface area of 3 to 30 m2/g, preferably 3 to 20 m2/g, more preferably 8 to 15 m2/g, and comprising at least 900% of a powder having a diameter of 0.2 to 0.7 μm. Examples of the crystalline growth-inhibitors are described to include ammonium salts and alkylammonium salts of citric acid, malic acid, adipic acid, gluconic acid, glucaric acid, glucuronic acid, tartaric acid and maleic acid.
Patent Publication 2 (Japanese Patent Provisional Publication 2004-59372) discloses a process for preparing a fine barium carbonate powder which comprises processing a mixture of a barium carbonate slurry and a granular medium in a fluid condition at a high rate, preferably in the presence of a particle growth-inhibitor such as a polyhydric alcohol, ascorbic acid, pyrophosphoric acid, carboxylic acid, or carboxylate. Patent Publication 2 describes that the disclosed process can give a barium carbonate powder having a BET specific surface area of 5 to 50 m2/g and a mean diameter (determined by a laser diffraction method) of 0.01 to 1.0 μm. Examples of the carboxylic acids and carboxylates employable as the particle growth-inhibitor are described to include citric acid, carboxymethylcellulose, oxalic acid, malonic acid, succinic acid, malic acid, maleic acid, tartaric acid, adipic acid, acrylic acid, polycarboxylic acid, polyacrylic acid, and their salts with sodium or ammonium.
Patent Publication 3 (Japanese Patent Provisional Publication 2006-206425) discloses a process for preparing a fine alkaline earth metal carbonate powder. The process comprises the steps of pulverizing an alkaline earth metal powder and preferably an aqueous solution containing at least one dispersant selected from the group consisting of a polycarboxylic acid having a weight average molecular weight of 1,000 to 20,000 and its salt under a wet condition and spray-drying the obtained dispersion. Patent Publication 3 describes that the disclosed process can give powdery calcium carbonate having a BET specific surface area of 10 to 150 m2/g, a primary particle size of which is in the range of 10 to 80 nm.
Patent Publication 4 (Japanese Patent Provisional Publication 11-1321) discloses a process for preparing a fine titanium dioxide powder which comprises the steps of dissolving titanyl sulfate in a mixture of water and an alcohol and heating the resulting solution under reflux. Patent Publication 4 describes that the disclosed process can give a titanium dioxide powder having a mean diameter of a nano order (in the range of 5.5 to 12.0 nm).
As is described hereinbefore, it is necessary to prepare fine ceramic materials such as a fine barium carbonate powder, a fine strontium carbonate powder, a fine calcium carbonate powder, and a fine titanium dioxide powder for producing a fine dielectric ceramic powder such as a fine barium titanate powder, a fine strontium titanate powder or a fine barium calcium titanate powder.
Generally, the dielectric ceramic powder is produced in industry by mixing the starting material powders under wet conditions. Therefore, it is preferred that the starting material powders can be dispersed in an aqueous medium to give a dispersion containing essentially primary particles by means of industrially employable dispersing procedures.
As is described above, it has been known that a very fine titanium dioxide powder can be prepared. However, the strontium carbonate powder and barium carbonate powder disclosed in Patent Publication 1 have a relatively large particle diameter.
Further, as is described in Patent Publications 2 and 3, a very fine alkaline earth metal carbonate powder can be obtained by pulverizing an alkaline earth metal carbonate powder in an aqueous medium utilizing a granular medium. There is problem, however, in that a dried powder is firmly aggregated due to van der Waals force and not easily re-dispersed in an aqueous solvent to give a dispersion containing the very fine powder, if the obtained fine powder in an aqueous medium is once dried.